Thermographic imaging process



Uni

3,37%,l27 Patented Apr. 23, 1968 3,379,127 THERMOGRAPHIC IMAGING PROCESS Douglas A. Newman, Glen Cove, N.Y., assignor to Q- lumbia Ribbon and Carbon Manufacturing Co., Inc, Glen Cove, N.Y., a corporation of New York No Drawing. Filed .luly 29, 1963, Ser. No. 298,402 6 Claims. (Cl. 101-471) This invention relates to improvements in the thermographic transfer method of imaging flexible copy sheets and masters.

According to the known thermographic transfer methods of imaging copy sheets, planographic printing masters and hectograph masters directly from original infrared radiation-absorbing images, the original images are superposed with a transfer sheet and a copy or master sheet and infrared radiation is applied to the original images wherein it is absorbed, causing the images to become heated. The images conduct a heat pattern to the transfer sheet causing the transfer composition to soften and adhere to the copy sheet or master in the image areas. Upon separation of the copy or master sheet and the transfer sheet, the copy or master sheet carries visible and/or duplicating images corresponding to the images on the original sheet.

Due to the requirement that the transfer sheet must be substantially free of materials which absorb infrared radiation, and the requirement that the transfer composition thereon must be so slightly bonded to the foundation as to release substantially completely therefrom in the course of the thermographic process, it has been found that thin plastic films function exceptionally well as transfer sheet foundations for thermographic use.

However, while film-base transfer sheets have the inherent properties to provide excellent transfer results, they do give rise to new problems due to the tendency of the plastic film to crimp and wrinkle in the heated areas. The failure of the transfer sheet to adhere in flat and continuous manner to the copy or master sheet in the heated areas causes the formation of duplicate images which are spotty or have void lines or snakes running therethrough, as discussed more fully in my copending application, Ser. No. 162,605, filed Dec. 27, 1961, now abandoned.

It is an object of the present invention to provide an improved thermographic imaging process which employs film-base transfer sheets which do not crimp or wrinkle in the heated image areas, the process resulting in the heat transfer of duplicate images which are sharp and uniform and free of void lines or discontinuities.

It is an advantage of this invention that the present film-base transfer sheets and units containing the same are easier to handle and work with than film-base transfer sheets heretofore known so that the present process is also more convenient than other known processes.

These and other objects and advantages will be apparent to those skilled in the art in light of the following description,

According to the present invention I have discovered that the tendency of film-base transfer sheets to crimp and wrinkle in the heated image areas may be substantially completely overcome by treating the plastic film, either before or after applying the transfer layer, with a coating of an anti-static agent which is free of materials which absorb substantial amounts of infrared radiation. It is not understood how or why such a treatment overcomes the problem of crimping or wrinkling of the film in the heated areas since the presence of static in the film has the advantage of causing the film to adhere strongly to the copy sheet or master in contact therewith. One possible explanation may be that the strong attraction between the static film and the copy sheet or master results in the trapping of air and such air forms minute pockets which prevent the transfer sheet and copy or master sheets from making perfect overall contact with each other.

No particular criticality resides in the selection of the particular films used as the foundations of the present transfer sheets aside from the requirement that the film must not absorb infrared radiation and must be sufficiently thin so as not to diffuse the heat pattern generated therein. It is preferred that the film have a maximum thickness of about 3 mils and a minimum thickness of about 0.3 mil. Suitable films include polyethylene terephthalate polyester (Mylar), polyethylene, polypropylene, chlorinated rubber, polytetrafiuoroethylene (Teflon), polyvinyl chloride, cellulose acetate, and the like.

The anti-static treatment preferably comprises a solvent-applied coating containing the anti-static agent and a binder material. The coating is preferably applied in such amounts as to form a thin anti-static film having a weight of from about 0.01 to about 0.10 ounce perthousand feet of per inch width of film but variations are possible depending upon the nature of the specific ingredients of the coating.

The following examples illustrate suitable anti-static coating solutions:

Example 1 Parts by weight Solid Glycol 6000Carbowax 2.0

Stearamidopropyldimethyl B hydroxyethyl-ammonium nitrate (dry basis)Catanac SN 1.0

Solids-sodium salt of the sulfonic acid of dioctyl succinic ester-Aerosol OT 0.1 Anhydrous denatured ethyl alcohol 96.9

Example 2 Solid Glycol 20M-Carbowax 2.0 Catanac SN 1.0 Aerosol OT 0.1 Water 5.0 Anhydrous denatured ethyl alcohol 91.9

100.0 Example 3 Ethyl cellulose 10.0 Catanac SN 7.6 Aerosol OT 0.4 Methyl ethyl ketone 82.0

The coating solution is applied to the uncoated surface of the film by flooding it on and removing the excess by a. blade. The volatile solvent is then driven off first by an air blast from a fan which causes practically all of the solvent to be evaporated. The coated film is then run over a hot roll, driving off the remainder of the solvent and fusing together the solid portions of the formula the hot roll being at such a temperature as to melt the residue of the coating and allow it to fuse into a continuous but very thin hard film.

While solid glycols of lower molecular weights, such as 900 for example, are somewhat hygroscopic and thus act to drain off any charge of static electricity which might accumulate, such coatings are relatively soft and melt at relatively low temperatures. They are not suitable as coatings to withstand the ambient heat encountered in the thermographic process particularly when radiation machines such as a Thermo-Fax machine are used. On the other hand, solid glycols having a molecular weight in the range of 4,000 to 20,000 produce higher melting point films having a minimum melting temperature of about 120 F. in the case of Carbowax 4000 and higher in the case of Carbowax 6000 and 20,000.

It is a requirement of the present invention that the anti-static coating be one which does not melt at the ambient temperatures encountered in the thermographic process and apparatus. Such melting results in a heattransfer of the coating to the sheet in contact therewith or to the belt or rollers or glass frame of the apparatus in cases where the coating makes contact therewith.

According to the preferred embodiment of the invention, the anti-static coating is one which does not melt and transfer even in the areas corresponding to the location of the heat-generating original images, such a composition being illustrated by Example 3. While it is ditficult to ascertain the degree of the heat in such areas, it appears that coatings having a melting temperature of 200 F. and higher have the necessary heat stability to resist melting and transfer in the heated image areas.

Suitable heat-resistant anti-static coatings are those based upon film-forming binder materials having a melting temperature of about 250 F. and higher. Illustrative of film-forming materials which may be used are ethyl cellulose, methyl cellulose, ethyl hydroxyethyl cellulose, polyvinyl chloride and polyvinyl acetate and copolymers thereof, polyvinyl butyral, vinylidene chloride polymers and copolymers, polystyrene, polyethylene and the like.

While it is preferred that the anti-static coating be non-melting at the temperatures generated in the heated image areas the present invention also contemplates the use of coatings which are meltable in such areas but which are not meltable at the ambient temperatures encountered in the thermographic process. Such coatings may be based upon the higher melting point Carbowaxes, or upon other hard waxes such as carnauba wax or mixtures of wax together with natural or synthetic resins. Such coatings must be non-melting at temperatures below about 120 F. and preferably are non-melting at temperatures below about 150 F.

It has been found that by the addition of infrared radiation-immune anti-static agents such as Catanac SN, the hard Carbowax coatings and coatings based upon film-forming binder material may 'be made readily conductive of static charges without appreciably softening the coating so that a non-greasy dry-feel film is produced. While Catanac has been found to provide excellent antistatic properties to the coating other conventional antistatic agents such :as succinonitrile, magnesium nitrate and other similar ionic agents may be used provided that they are permeable and do not absorb infrared radiation to any substantial degree.

The anti-static coating may be applied either before or after the application of the transfer coating and may be applied to one or both sides of the film foundation. However, it is preferable to apply the anti-static coating first thereby avoiding the reheating of the transfer coating during the operation of drying the anti-static coating. The anti-static coating is preferably applied on only one side of the film foundation and the heat-transferable layer is applied to the opposite surface of the foundation. If desired, some antistatic compound may also be included in the transferable imaging layer.

The foregoing examples are given by way of illustration and should not be considered limitative. Other conventional anti-"static agents and wax and resinous binder materials may be used in place of those set out in the examples, the essential requirements being that they do not absorb substantial amounts of infrared radiation and that they form anti-static films which are not meltable at the ambient temperatures generated in the thermographic process, i.e., at temperatures below about F.

The heat-transferable coatings on the present transfer sheets may be selected from any of the conventional coatings known for this purpose but must be heat-meltable at temperatures below the melting temperature of the particular anti-static coating used in conjunction therewith and above the ambient temperatures encountered. Thus both the anti-static and imaging coatings must be meltable above about 120 F. but the anti-static coating must have at least a slightly higher melting temperature than the imaging coating. When the transfer layer is used for the imaging of single copy sheets, it may consist of a small amount of dissolved dyestuif or pigment carried by a wax binder material. For the imaging of hectograph master sheets, the transfer layer may consist of large amounts of undissolved particulate hectograph dyestuffs carried by a wax or resinous binder material. Oleophilic transfer layers, preferably wax tinted lightly for proofreading purposes, are used for the imaging of paper or plastic film planographic printing masters or plates.

The present heat-transferable coatings 0r layers are substantially free of materials which absorb infrared radiation and are applied to the thin film foundations in the form of exceptionally thin layers preferably having a caliper ranging as thin as from about 0.2 mil to about 0.6 mil.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. In the process of imaging a flexible sheet comprising the steps of superposing original infrared radiationabsorbing images together with a flexible sheet and a transfer sheet having a heat-transferable imaging layer, the imaging layer being in intimate surface contact with the surface of the flexible sheet, and applying to said images radiations rich in infrared which are absorbed by the images and heat the imaging layer and cause it to melt and transfer to the flexible sheet in image form in areas corresponding to the location of the original images, the improvement which comprises using as the transfer sheet one having a thin plastic foundation having on the surface thereof opposite the surface carrying the heattransferable imaging layer a thin anti-static coating which is free of materials which absorb infrared radiation to any substantial degree, said coating having a melting point of at least 120 P. so as to resist melting at the amibent temperatures of the said process, whereby the tendency of the transfer sheet to crimp and wrinkle in the heated areas is avoided.

2. The process according to claim 1 in which the flexible sheet is a hectograph master sheet and the heattransferable imaging layer contains hectograph imaging material.

3. The process according to claim 1 in which the flexible sheet is a planographic printing plate and the heattransferable imaging layer contains oleophilic imaging material.

4. The process according to claim 1 in which the anti static coating comprises an anti-static agent dispersed in a binder material.

5. The process according to claim 4 in which the binder material is a film-forming binder material.

6. The process of claim 5 in which the film-forming binder material comprises ethyl cellulose.

References Cited UNITED STATES PATENTS McDaniel 3172 X Speicher 317-2 X Roshkind 101149.2 X Peshin et a1 1173 6.4

6 3,122,998 3/1964 R aczynski et a1. 101-1494 X 3,131,080 4/1964 Russell l01149.4 X

FOREIGN PATENTS 722,023 1/1955 Great Britain. 

1. IN THE PROCESS OF IMAGING A FLEXIBLE SHEET COMPRISING THE STEPS OF SUPERPOSING ORIGINAL INFRARED RADIATIONABSORBING IMAGES TOGETHER WITH A FLEXIBLE SHEET AND A TRANSFER SHEET HAVING A HEAT-TRANSFERABLE IMAGING LAYER, THE IMAGING LAYER BEING IN INTIMATE SURFACE CONTACT WITH THE SURFACE OF THE FLEXIBLE SHEET, AND APPLYING TO SAID IMAGES RADIATIONS RICH IN INFRARED WHICH ARE ABSORBED BY THE IMAGES AND HEAT THE IMAGING LAYER AND CAUSE IT TO MELT AND TRANSFER TO THE FLEXIBLE SHEET IN IMAGE FORM IN AREAS CORRESPONDING TO THE LOCATION OF THE ORIGINAL IMAGES, THE IMPROVEMENT WHICH COMPRISES USING AS THE TRANSFER SHEET ONE HAVING A THIN PLASTIC FOUNDATION HAVING ON THE SURFACE THEREOF OPPOSITE THE SURFACE CARRYING THE HEATTRANSFERABLE IMAGING LAYER A THIN ANTI-STATIC COATING WHICH IS FREE OF MATERIALS WHICH ABSORB INFRARED RADIATION TO ANY SUBSTANTIAL DEGREE, SAID COATING HAVING A MELTING POINT OF AT LEAST 120*F. SO AS TO RESIST MELTING AT THE AMIBENT TEMPERATURES OF THE SAID PROCESS, WHEREBY THE TENDENCY OF THE TRANSFER SHEET TO CRIMP AND WRINKLE IN THE HEATED AREAS IS AVOIDED. 